Re: [PATCH] drm/i915: Rewrite vlv_find_best_dpll()

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Sep 23, 2013 at 09:03:10PM +0300, ville.syrjala@linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> vlv_find_best_dpll() has several integer over/underflow issues,
> includes a hand rolled DIV_ROUND_CLOSEST(), has a boat load of
> variables, some slightly weird math, and it doesn't look very
> nice either.
> 
> Rather than try to deal with each issue separately I just decided
> to rewrite the function a bit.
> 
> WARNING: Entirely untested
> 
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_display.c | 94 +++++++++++++++---------------------
>  1 file changed, 40 insertions(+), 54 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 3b06250..f89fb12 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -670,65 +670,51 @@ vlv_find_best_dpll(const intel_limit_t *limit, struct drm_crtc *crtc,
>  		   int target, int refclk, intel_clock_t *match_clock,
>  		   intel_clock_t *best_clock)
>  {
> -	u32 p1, p2, m1, m2, vco, bestn, bestm1, bestm2, bestp1, bestp2;
> -	u32 m, n, fastclk;
> -	u32 updrate, minupdate, p;
> -	unsigned long bestppm, ppm, absppm;
> -	int dotclk, flag;
> -
> -	flag = 0;
> -	dotclk = target * 1000;
> -	bestppm = 1000000;
> -	ppm = absppm = 0;
> -	fastclk = dotclk / (2*100);
> -	updrate = 0;
> -	minupdate = 19200;
> -	n = p = p1 = p2 = m = m1 = m2 = vco = bestn = 0;
> -	bestm1 = bestm2 = bestp1 = bestp2 = 0;
> +	intel_clock_t clock = {
> +		.dot = target * 5, /* fast clock */
> +	};
> +	unsigned int bestppm = 1000000;
> +	/* min update 19.2 MHz */
> +	int max_n = min(limit->n.max, refclk / 19200);
>  
>  	/* based on hardware requirement, prefer smaller n to precision */
> -	for (n = limit->n.min; n <= ((refclk) / minupdate); n++) {
> -		updrate = refclk / n;
> -		for (p1 = limit->p1.max; p1 > limit->p1.min; p1--) {
> -			for (p2 = limit->p2.p2_fast+1; p2 > 0; p2--) {
> -				if (p2 > 10)
> -					p2 = p2 - 1;
> -				p = p1 * p2;
> -				/* based on hardware requirement, prefer bigger m1,m2 values */
> -				for (m1 = limit->m1.min; m1 <= limit->m1.max; m1++) {
> -					m2 = (((2*(fastclk * p * n / m1 )) +
> -					       refclk) / (2*refclk));
> -					m = m1 * m2;
> -					vco = updrate * m;
> -					if (vco >= limit->vco.min && vco < limit->vco.max) {
> -						ppm = 1000000 * ((vco / p) - fastclk) / fastclk;
> -						absppm = (ppm > 0) ? ppm : (-ppm);
> -						if (absppm < 100 && ((p1 * p2) > (bestp1 * bestp2))) {
> -							bestppm = 0;
> -							flag = 1;
> -						}
> -						if (absppm < bestppm - 10) {
> -							bestppm = absppm;
> -							flag = 1;
> -						}
> -						if (flag) {
> -							bestn = n;
> -							bestm1 = m1;
> -							bestm2 = m2;
> -							bestp1 = p1;
> -							bestp2 = p2;
> -							flag = 0;
> -						}
> -					}
> -				}
> +	for (clock.n = limit->n.min; clock.n <= max_n; clock.n++) {
> +	for (clock.p1 = limit->p1.max; clock.p1 > limit->p1.min; clock.p1--) {
> +	for (clock.p2 = limit->p2.p2_fast+1; clock.p2 > 0; clock.p2--) {

I think that's going to upset the coding style police ;-) I guess it would
be simple to extract a vlv_compute_clock like we have for pnv/i9xx that's
both used here and in the get_clock code from Jesse.
-Daniel


> +		if (clock.p2 > 10)
> +			clock.p2--;
> +		clock.p = clock.p1 * clock.p2;
> +
> +		/* based on hardware requirement, prefer bigger m1,m2 values */
> +		for (clock.m1 = limit->m1.min; clock.m1 <= limit->m1.max; clock.m1++) {
> +			unsigned int ppm, diff;
> +
> +			clock.m2 = DIV_ROUND_CLOSEST(clock.dot * clock.p * clock.n,
> +						     clock.m1 * refclk);
> +			clock.m = clock.m1 * clock.m2;
> +
> +			clock.vco = refclk * clock.m / clock.n;
> +
> +			if (clock.vco < limit->vco.min ||
> +			    clock.vco >= limit->vco.max)
> +				continue;
> +
> +			diff = abs(clock.vco / clock.p - clock.dot);
> +			ppm = div_u64(1000000ULL * diff, clock.dot);
> +
> +			if (ppm < 100 && clock.p > best_clock->p) {
> +				bestppm = 0;
> +				*best_clock = clock;
> +			}
> +
> +			if (ppm + 10 < bestppm) {
> +				bestppm = ppm;
> +				*best_clock = clock;
>  			}
>  		}
>  	}
> -	best_clock->n = bestn;
> -	best_clock->m1 = bestm1;
> -	best_clock->m2 = bestm2;
> -	best_clock->p1 = bestp1;
> -	best_clock->p2 = bestp2;
> +	}
> +	}
>  
>  	return true;
>  }
> -- 
> 1.8.1.5
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch